The present invention relates to a high voltage direct current (DC) cable having an impregnated stratified insulation. More particularly, the present invention relates to a high voltage DC cable having a stratified insulation made from a paper-polypropylene laminate impregnated with an electrically insulating fluid, said cable being suitable for terrestrial or, preferably, submarine installations.
For the purposes of the present description and the appended claims, with the term “high voltage” it is meant a voltage of at least 35 kV. For the purposes of the present description and the appended claims, with the term “very high voltage” it is meant a voltage of at least 200 kV, preferably of at least 300 kV.
Cables with impregnated stratified insulation are known wherein the electrical conductor is electrically insulated by winding thin tapes made from paper or, preferably, from a multilayer paper-polyolefin (typically polypropylene) laminate. The stratified insulation is then thoroughly impregnated with a fluid having high electrical resistivity and a predetermined viscosity, the importance of which will be discussed in the following.
As reported, for example, by U.S. Pat. No. 6,207,261, examples of DC (direct current) and AC (alternate current) impregnated power cables include:
self-contained oil filled (OF) cable impregnated with insulating oil having a relatively low viscosity, supplied from an oil feeding apparatus provided at one or both ends of the cable line so that the insulating layer is kept under a positive pressure by the insulating oil;
high-pressure pipe-type OF (POF) cable deployed by inserting a cable core (assembly of cable conductor/s and insulation) into a steel pipe previously installed, evacuating the steel pipe and filling the steel pipe with an insulating oil having a slightly higher viscosity than that of insulating oil for OF cable;
mass-impregnated (or solid) cable being impregnated with an insulating oil having a higher viscosity than that of insulating oil for POF cable, covered with a metallic sheath.
High voltage direct current (HVDC) mass-impregnated cables are especially useful for long distance power transportation, especially along submarine lines, as from, e.g., U.S. Pat. No. 4,782,194 or WO 99/33068. Besides the advantages provided by the direct current transportation (e.g. with consistently reduced dielectric losses), the HVDC cables do not suffer from fluid migration encountered in mass impregnated HVAC (high voltage alternate current) cables. Oil-impregnated HVAC cables are usually of the above mentioned OF or POF type.
GB 2,196,781 discloses compositions known for impregnating layered insulation for DC cable have a viscosity, at room temperature (20° C.), of from 1000 to 50000 cSt.
The step of impregnating the paper-polypropylene laminate with the fluid is critical. In particular, the semifinished laminated cable core is submerged into the fluid and left to stand for a period typically lasting about 30 days to allow the fluid to penetrate even into the most radially inner layers of laminate. A full and complete penetration of the fluid is of the utmost importance for avoiding a significant reduction of the electrical performance. During impregnation the laminate swells to some extent, the phenomenon being mainly due to the swelling of the polypropylene layer. Such swelling could cause delamination. The possible separation of one layer from the other, even if partial, has extremely serious consequences on the functionality of the cable. Efforts have been made for improving the adhesion between paper and polypropylene to obtain a laminate with an improved resistance to swelling. Features like paper density and permeability, polypropylene cristallinity, special treatment in the manufacturing of the laminate were considered.
The use of a high viscosity fluid, as generally employed for mass-impregnated cables, makes the impregnation process even more critical, as explained hereinunder.
U.S. Pat. No. 5,850,055 relates to an electrical cable for high and very high voltages wherein the conductors are surrounded by a stratified insulation impregnated with an insulating fluid, said insulation being constituted by a paper/polypropylene/paper laminate wherein the central layer is formed by a radiated polypropylene film, i.e. a polypropylene film radiated with high-energy ionizing radiations. The insulating fluid is an oil having a very low viscosity, of the order of 5-15 centistokes, and a resistivity of at least 1016 ohm/cm, such as mineral oils, alkyl naphthalenes and alkyl benzenes. The paper has a low density, typically a maximum density of 0.85 g/cm3, preferably from 0.65 to 0.75 g/cm3. Typically, the paper has an impermeability to air ranging from 10×106 to 30×106 Emanueli units (corresponding to (Gurley unit×455)/paper thickness (mm)).
The cable disclosed by the above mentioned patent is impregnated with a low viscosity oil that is not suitable for mass-impregnated cables.
U.S. Pat. No. 6,207,261 relates to an electrical insulating laminated paper comprising one or two sheets of a kraft insulating paper and a plastic film layer of a polyolefin resin integrated by melt extrusion, which has been calendered or supercalendered, whereby the total thickness thereof is from 30 to 200 μm and the proportion of the plastic film layer is from 40 to 90%. Examples of laminates comprising paper with a density of 0.70-0.72 g/cm3 and an air impermeability of 2,500-3,000 sec/100 ml are compared with laminates comprising paper with a density of 1.09-1.13 g/cm3 and an air impermeability of at least 100,000 sec/100 ml (corresponding to 100,000 Gurley sec−1). The laminates were subjected to ageing test at a temperature of 100° C. in an alkylbenzene oil (a low viscosity insulating oil) which is used in OF cable for 24 hours. After ageing, the adhesive strength between the paper layers and the polypropylene layer was measured: the comparative specimens using high density and high air impermeability paper showed a very poor adhesive strength and underwent complete peeling of the layers during or after dipping in the alkylbenzene oil.